Re: UNION ALL

Generally speaking, when executing UNION ; a DISTINCT is run afterward on the resultset.

So, if you're sure that each part of UNION cannot return a line returned by another one, you may use UNION ALL, you'll cut the cost of the final implicit DISTINCT.

----- Mail original -----
De: "Mark Pasterkamp" <markpasterkamp1994(at)gmail(dot)com>
À: pgsql-hackers(at)lists(dot)postgresql(dot)org
Envoyé: Jeudi 15 Août 2019 20:37:06
Objet: UNION ALL

Dear all,

I was wondering if someone could help me understands what a union all actually does.

For my thesis I am using Apache Calcite to rewrite queries into using materialized views which I then give to a Postgres database.
For some queries, this means that they will be rewritten in a UNION ALL style query between an expression and a table scan of a materialized view.
However, contrary to what I expected, the UNION ALL query is actually a lot slower.

As an example, say I have 2 tables: actor and movie. Furthermore, there is also a foreign key index on movie to actor.
I also have a materialized view with the join of these 2 tables for all movies <= 2015 called A.
Now, if I want to query all entries in the join between actor and movie, I would assume that a UNION ALL between the join of actor and movie for movies >2015 and A is faster than executing the original query..
If I look at the explain analyze part, I can certainly see a reduction in cost up until the UNION ALL part, which carries a respective cost more than negating the cost reduction up to a point where I might as well not use the existing materialized view.

I have some trouble understanding this phenomenon.
One thought which came to my mind was that perhaps UNION ALL might create a temporary table containing both result sets, and then do a table scan and return that result.

this would greatly increase IO cost which could attribute to the problem.
However, I am really not sure what UNION ALL actually does to append both result sets so I was wondering if someone would be able to help me out with this.

Mark